I filed for divorce in October, but have not completed the process. I just learned that my ex filed "Single" this year. How should I file?

[Phillip1 · Accepted solution]

He should have filed married filing separate. You should file married filing separate as well unless the following applies:

1. You and your spouse lived apart for more than the last 6 months of the year (from July 1).
2. There is a qualifying child since you and your spouse separated; and 
3. You would meet the qualifications to file head of household if you were not married.

See the following from IRS Publication 501:

Head of Household

You may be able to file as head of household if you meet all the following requirements.

1. You are unmarried or considered unmarried on the last day of the year. See Marital StatusConsidered Unmarried

2. You paid more than half the cost of keeping up a home for the year.

3. A qualifying person lived with you in the home for more than half the year (except for temporary absences, such as school). However, if the qualifying person is your dependent parent, he or she doesn't have to live with you. See Special rule for parent , later, under Qualifying Person.

Considered Unmarried

To qualify for head of household status, you must be either unmarried or considered unmarried on the last day of the year. You are considered unmarried on the last day of the tax year if you meet all the following tests.

1. You file a separate return. A separate return includes a return claiming married filing separately, single, or head of household filing status.

2. You paid more than half the cost of keeping up your home for the tax year.

3. Your spouse didn't live in your home during the last 6 months of the tax year. Your spouse is considered to live in your home even if he or she is temporarily absent due to special circumstances. See Temporary absences , later.

4. Your home was the main home of your child, stepchild, or foster child for more than half the year. (See Home of qualifying person , later, for rules applying to a child's birth, death, or temporary absence during the year.)

5. You must be able to claim an exemption for the child. However, you meet this test if you can't claim the exemption only because the noncustodial parent can claim the child using the rules described later in Children of divorced or separated parents (or parents who live apart) under Qualifying Child or in Support Test for Children of Divorced or Separated Parents (or Parents Who Live Apart) under Qualifying Relative. The general rules for claiming an exemption for a dependent are explained later under Exemptions for Dependents .